1. Field of Invention
The invention relates, generally, to a composite structural element and, particularly, to such an element for use in a vehicle suspension as well as a method for manufacturing the element.
2. Description of Related Art
The materials generally used in mass production of components for vehicle suspensions, such as motorcar suspensions, are steel, cast iron, and aluminium for reasons due to costs of production and performances of the material (weight, stiffness, “fatigue” life, resistance to limit load conditions, etc.). The use of composite materials for the production of motorcar-suspension components is traditionally limited to racing cars, such, as the “Formula 1” cars or so-called “supercars,” as in this case the weight and performance requirements are of higher importance than the cost.
In the design of components for vehicle suspensions (in particular, for motorcar suspensions), a number of requirements conflicting with each other must be complied with. In particulars, a motorcar-suspension component must be capable of bearing certain kinds of loads (the so-called “fatigue loads”), representative of the normal use of the vehicle, although in heavy conditions. These loads are applied alternately onto the component, and this latter must not suffer from crack formation or failures within a given number of “fatigue” cycles applied. High-strength materials, such as fibre-based composite materials, are suggested in order to fulfil this “fatigue life” requirement and to limit, at the same time, the weight. Another structural requirement that must be fulfilled by the motorcar-suspension component is that the component must be able to deform in a predictable and particular way under so-called “misuse loads” (i.e., under “limit load” conditions). With such a kind of stresses, the component must be able to deform, reacting with a given reaction load and absorbing a given amount of energy, but connection between wheel and vehicle must always be ensured. In particular, any possible failure must be confined to given zones and must not occur below a given amount of deformation. Ductile materials, such as steel, are suggested in order to fulfil these requirements for controlled deformation and for presence of a “deformation witness” mark under “limit load” conditions.
U.S. Pat. No. 7,159,880 discloses a structural element (in particular, for a vehicle chassis) comprising an elongated body and a pair of connection members mounted at the opposite ends of the elongated body, wherein the elongated body consists of a metal core on which fibre-reinforced, plastic material is over-moulded by injection moulding so as to provide the elongated body with a cross-section having the desired shape. On the one hand, the use of a fibre-reinforced plastic material allows to limit the weight of the component and to ensure, at the same time, high mechanical properties while, on the other hand, the use of a metal core avoids the loss of functionality of the component even in case of damage. Such a known solution is, however, affected by the drawback that the use of the over-moulding technique to produce the plastic material portion of the structural element makes it possible to obtain only structural elements with a solid cross-section. This inevitably involves limits to the freedom of the designer in designing the cross-section of the element, which limits are excessively penalizing, for instance, in case of structural elements intended to be used for triangular suspension arms. As is known, in order to increase the strength of the structural element, it is necessary to increase the moment of inertia of the cross-section thereof [in other words, to shift the material of the cross-section as far as possible from the middle plane thereof (in the present case, from the metal core), which results in an excessive increase in the overall weight of the element].
Further examples of structural elements comprising a metal core on which plastic material (if necessary, reinforced with fibres) is over-moulded are known from U.S. Pat. No. 6,030,570 and International Patent Application Publications WO2003/039893 and WO2003/039892. Also, these structural elements suffer from the same drawback discussed above with reference to U.S. Pat. No. 7,159,880.
U.S. Patent Application Publication 2004/0131418 discloses a structural element comprising a first part of metal having a U-shaped cross-section and a second part of plastic material that is attached to the first part to close the U-shaped cross-section thereof, thus forming a hollow structural element having a closed cross-section. The first and second parts are obtained separately from each other and are then joined to each other by bending the edge of the second part onto the edge of the first part with the use of heat or ultrasounds. On the one hand, this known solution offers the advantage of reducing the overall weight of the structural element due to one of the two parts of which the element consists being made of plastic material instead of metal. On the other hand, the weight reduction allowed by this known solution is minimum, as the cross-section of the structural element is mainly formed by the first part (i.e., by the metal part).
It is, therefore, an object of the invention to provide a composite structural element that is able to offer similar or better performances with respect to the prior art with a smaller weight.